"Smart Pixel" is a term applied to a device that includes one or more input optical receivers, one or more output optical modulators (or lasers), and electronic logic circuitry. Smart pixels are used in the field of optical interconnection within and between digital computing systems, such as switching systems and parallel processors. In one implementation of this concept, large numbers of optical receivers and transmitters are directly integrated with semiconductor electronic processing elements directly on the semiconductor chips that form the transmitter and/or receiver circuits. This has a number of potential advantages for switching and computing systems as detailed in the literature. See J. W. Goodman, F. I. Leonberger, Sun-Yuan Athale, R. A. Kung, Proceedings of the IEEE, 72,850 (Jul. 1984), and F. B. McCormick, et al, Applied Optics, 33, pp. 1601-1618, 1994.
The local nature of a smart pixel system may permit it to employ a receiver that operates more than one beam. A two-beam receiver provides advantages, such as increased noise immunity, because of the use of relative referencing, that is, using a signal corresponding to the difference between the signals from the two beams as the one to be processed. Optical receivers designed to operate with more than one optical beam are described in U.S. Pat. No. 5,389,776 to Woodward, granted Feb. 14, 1995 and U.S. Pat. No. 5,581,077 to Chirovsky, Novotny and Woodward, granted Dec. 3, 1996, both of which are assigned to the assignee of this application.
A common optical receiver design uses a trans-impedance amplifier. The feedback element of the trans-impedance amplifier stage is critical to the operation of the receiver. Use of a micro-FET (field effect transistor device) has been proposed as a feedback element in a trans-impedance amplifier, for example, as described in Gareth F. Williams, "Lightwave Receivers", in Topics in Lightwave Systems, ed. Tingye Li. (Academic Press, 1991), pp. 79-149. ISBN 0-12-447302-4. The micro-FET has the advantage of permitting a high-value resistance to be obtained without the penalty of high parasitic capacitance, as would occur with a conventional resistor. A trans-impedance amplifier using a single PFET device, or a PFET in combination with a diode-connected NFET, also has been used as a feedback element as described in Woodward, et al., IEEE Photo. Tec. Lett., Vol. 8, pp. 422-424, Mar. 1996.
A difficulty arises, however, when using a single FET as the resistance feedback element in an amplifier stage of a two-beam optical receiver. With an amplifier that has a quiescent output voltage state Ve in a two-beam receiver, the amplifier output signal swings both positively and negatively with respect to Ve. This is in contrast to a single beam receiver in which the output swings in only one polarity with respect to Ve. Since the resistance of a single FET used as the feedback element is not symmetric with respect to positive and negative signal swings, the amplifier will experience different closed loop gain for positive and negative excursions of the output signal about Ve, depending on whether the feedback element is an N or P type FET. It will be appreciated that such an asymmetric response is undesirable in a two-beam receiver configuration.
The asymmetric response is shown referring to FIG. 1 and FIGS. 2A and 2B. FIG. 1 shows a basic representation of a FET device with typical operating voltages Vg (gate), Vd (drain), Vs (source) and Vb (body terminal). FIG. 1 illustratively represents either an N conductivity type FET (NFET) or a P conductivity type FET (PFET). FIGS. 2A and 2B illustratively shows NFET and PFET current-voltage (I-V) characteristics and differential resistance for the case where there is a fixed gate bias Vg and a fixed body terminal bias Vb.
Since an amplifier of a typical optical receiver may have an output voltage that varies either above or below a quiescent bias point (Ve), and a feedback element is connected between the amplifier output and input, of interest is the current voltage (I-V) behavior of an FET in which one of the source and drain is held at a relatively fixed voltage while the other terminal is varied through the power supply range. The gate voltage is also held fixed typically at either the corresponding positive or negative supply voltage, although dedicated bias levels are also possible.
Referring to FIG. 2A, for an NFET device of FIG. 1, consider that Vg=5.2V, Vb=0V and source voltage Vs is held fixed at 2.2V. The drain terminal voltage Vd is varied from 0 to 5V. As seen in FIG. 2A, in one bias direction of Vd at about 3V, the NFET device enters saturation, that is, the 3V curve (solid line) flattens out, while in the other bias direction it does not. Thus, the differential resistance curve is not uniform, as shown by the dotted line, for values positive and negative from Vd=2.5V over the range 0 to 5 volts.
In FIG. 2B, for a PFET device, consider that Vg=0.2V and Vb=5V. As seen, the I-V curve flattens at a Vd of below about 2V and the differential resistance is not equal above and below Vd=2.5 as shown by the dotted line. The curves of FIGS. 2A and 2B are for FETs having a gate length of 0.8 .mu.m and a gate width of 1 .mu.m, but the basic device behavior is independent of these choices.
Since the resistance of the amplifier feedback element affects the smart pixel receiver overall operation, and the resistance of a single FET used as a feedback element is not symmetrical, for operation of a two beam optical receiver it is advantageous to provide a feedback element that has a more linear and symmetrical differential resistance characteristic.